Taking folic acid before and during early pregnancy is one of the few steps proven to lower the risk of serious birth defects of the brain and spine. The evidence comes from large clinical trials and from 25 years of tracking births in the U.S.

The short version

  • How much: 400 micrograms (mcg) of folic acid every day if you could become pregnant. The CDC recommends this even if you aren’t planning a pregnancy.
  • When: Start at least one month before you get pregnant, and keep taking it through the first 2 to 3 months of pregnancy, according to the U.S. Preventive Services Task Force.
  • Where: Most prenatal vitamins already contain it. Look for the words “folic acid” on the label.
  • Exceptions: If you’ve had a pregnancy affected by a neural tube defect, or you take certain medicines, the right amount is different. Talk with your doctor before you get pregnant.

Why the first few weeks matter most

The protection folic acid offers happens very early, often before you know you’re pregnant. That’s why the advice is to start before pregnancy, not after a positive test.

Folic acid is the form of folate (vitamin B9) used in vitamins and fortified foods. Your body needs folate to make new cells, and an early pregnancy makes new cells fast.

One of the first structures to form is the neural tube, which becomes the baby’s brain and spinal cord. It closes 21 to 28 days after conception, according to the NIH Office of Dietary Supplements. When it doesn’t close completely, the result is a neural tube defect (NTD). The two most common are:

  • Spina bifida, where the spine doesn’t close fully. It can cause problems with walking, bladder and bowel control, and learning.
  • Anencephaly, where much of the brain and skull doesn’t develop. Almost all babies with anencephaly die before or shortly after birth.

NTDs affect an estimated 3,000 pregnancies in the U.S. each year. About half of U.S. pregnancies are unplanned, so many women are already past the critical window by the time they find out.

What the research shows

In the 1990s, two large randomized trials showed that vitamins with folic acid, taken before and during early pregnancy, lowered the risk of NTDs. In one of them, a high dose cut repeat NTD pregnancies by 71% among women who had already had an affected pregnancy, as reported by the CDC.

Those findings led the U.S. to require folic acid in enriched grain products starting in 1998. Since then, NTD rates have dropped 28%. About 1,300 babies a year are now born without an NTD who would otherwise have had one, and spina bifida cases that do occur tend to be less severe, according to the CDC.

Two limits: folic acid lowers the risk, but it doesn’t prevent every NTD. And folic acid is the only form of folate that has been shown in studies to prevent them.

How much to take, and when

For most women, the answer is 400 mcg of folic acid a day, started at least a month before pregnancy. Your total folate needs then rise during pregnancy and breastfeeding.

Your situation What’s recommended Source
Could become pregnant 400 mcg of folic acid daily, from a vitamin, fortified foods, or both CDC
Planning a pregnancy A daily supplement with 400 to 800 mcg, starting at least 1 month before conception and continuing through the first 2 to 3 months USPSTF
Pregnant 600 mcg DFE of total folate a day from all sources (food plus vitamins) NIH
Breastfeeding 500 mcg DFE of total folate a day from all sources NIH
Had a pregnancy affected by an NTD 4,000 mcg a day, starting 1 month before pregnancy through the first 3 months, under a doctor’s care CDC

DFE stands for dietary folate equivalents, a unit explained in the label section below. ACOG’s version of the high-risk recommendation starts 3 months before conception and also covers women who have a family history of NTDs or whose partner does, according to the NIH’s summary of guidelines.

400 or 800? Both fall within the Task Force’s range, which reflects the doses commonly sold. More is not necessarily better for preventing NTDs, the CDC notes, unless your doctor recommends a higher amount.

Already pregnant and haven’t been taking it? Start now. Your body’s need for folate stays higher for the whole pregnancy, so it still matters after the first trimester. Tell your provider so they can factor it into your care.

When the standard dose may not be right for you

A few situations call for a conversation with your doctor before pregnancy, because the right amount may be higher or your medicines may need adjusting.

  • A past pregnancy affected by an NTD. The CDC’s recommendation is 4,000 mcg a day, ten times the usual amount, under a doctor’s care. Don’t try to reach it by taking extra multivitamins; you’d also get too much of the other vitamins they contain.
  • A family history of NTDs. Having had an NTD yourself, or having a close relative with one, raises the risk, according to the U.S. Preventive Services Task Force.
  • Medicines that affect folate. These include methotrexate and some anti-seizure medicines, such as carbamazepine and valproic acid. The standard recommendation doesn’t apply to people taking them, according to the Task Force. Folic acid can also change how some anti-seizure medicines work, so don’t start, stop, or change a dose on your own.
  • Diabetes or obesity. Both are linked to a higher risk of NTDs, according to the NIH. Ask whether your plan should change.
  • Weight-loss surgery or digestive conditions. Conditions like celiac disease or inflammatory bowel disease can make it harder to absorb folate.

Hispanic women have the highest rate of spina bifida in the U.S., according to the CDC. Since 2016, corn masa flour, used for tortillas and tamales, can be fortified with folic acid. Look for “folic acid” on the package.

Food, fortified foods, and vitamins

A folate-rich diet is good for you, but most women need a vitamin or fortified foods to reliably get 400 mcg of folic acid. Two reasons:

  1. Your body absorbs folic acid better. It absorbs at least 85% of the folic acid in a vitamin or fortified food taken with a meal, but only about half of the folate found naturally in food, according to the NIH.
  2. Many women rely on one source. About half of U.S. women say enriched grains are their only source of folic acid, according to the CDC. Skip bread and pasta for a while, and that source drops off.

That’s why most providers recommend a daily prenatal vitamin alongside a healthy diet. The Dietary Guidelines for Americans note this matters most for folate, iron, iodine, and vitamin D.

Good food sources of folate

These still count toward your total folate need, and they bring fiber and other nutrients a vitamin can’t.

Food Serving Folate (mcg)
Spinach, boiled ½ cup 131
Black-eyed peas, boiled ½ cup 105
Breakfast cereal fortified with 25% of the Daily Value 1 serving 100
Asparagus, boiled 4 spears 89
Brussels sprouts, cooked ½ cup 78
Romaine lettuce, shredded 1 cup 64
Avocado, sliced ½ cup 59
White rice, enriched, cooked ½ cup 54
Broccoli, cooked ½ cup 52
Kidney beans, canned ½ cup 46
Orange juice ¾ cup 35

Values from the NIH Office of Dietary Supplements. Cereal and rice values are folic acid added through fortification.

Reading your prenatal vitamin label

What you’re looking for is the word “folic acid” and an amount of at least 400 mcg. Three things on today’s labels tend to confuse people.

Why the number looks bigger than 400

Newer labels list folate in mcg DFE (dietary folate equivalents). DFE adjusts for how much better your body absorbs folic acid than food folate, according to the NIH. The adjustment is roughly 1.7 times, so 400 mcg of folic acid usually appears as about 680 mcg DFE. Many labels show both, like “680 mcg DFE (400 mcg folic acid).”

Folate, folic acid, or methylfolate?

“Folate” is the umbrella name for all forms of vitamin B9. Some prenatal vitamins use a form called methylfolate (5-MTHF) instead of folic acid. Folic acid is the only form shown in studies to help prevent NTDs, according to the CDC. If your vitamin lists only methylfolate, ask your provider whether it meets your needs. This isn’t rare: about a quarter of over-the-counter prenatal vitamins and about a third of prescription ones use methylfolate, according to a 2020 evaluation summarized by the NIH.

What about the MTHFR gene?

You may have read that people with an MTHFR gene variant can’t use folic acid. The CDC says that’s not true:

  • MTHFR variants are common. More people in the U.S. carry one or two copies of the most common variant than don’t.
  • People with these variants can process folic acid. Even with two copies, blood folate levels are only about 16% lower, on average, than in people without the variant.
  • How much folic acid you take, and for how long, matters more than your MTHFR type.

The American College of Medical Genetics and Genomics advises against routine MTHFR testing, and says MTHFR status doesn’t change the recommendation to take the standard dose of folic acid.

Look for independent testing

What’s in the bottle doesn’t always match the label. In a 2023 federal review of 12 over-the-counter prenatal vitamins, 11 had at least one nutrient outside acceptable limits, according to the NIH. A product tested by an independent group such as U.S. Pharmacopeia (USP), NSF, or ConsumerLab is more likely to contain what the label says.

Common questions

Is folic acid safe in pregnancy? Yes, at the usual doses. The Task Force found that the harms of 400 to 800 mcg a day are no greater than small.

Can I take too much? For adults, including during pregnancy, the upper limit for folic acid from vitamins and fortified foods is 1,000 mcg a day; for teens 14 to 18, it’s 800 mcg. There’s no upper limit for folate from regular food. Higher doses should only be taken under a doctor’s care, according to the NIH. Check that you aren’t doubling up, for example by taking a prenatal vitamin and a separate folic acid pill. Going over isn’t rare: prenatal vitamins contain a median of 800 mcg of folate, and in national survey data from the early 2000s, about a third of pregnant women got more supplemental folate than the upper limit, according to the NIH.

Why does the upper limit exist? Large amounts of folic acid can hide the anemia caused by low vitamin B12 while B12-related nerve damage continues. This matters most if you eat little or no meat or dairy, since B12 comes mainly from animal foods. One study also linked supplement intakes of 1,000 mcg or more around conception with lower scores on children’s thinking-skills tests at ages 4 to 5. That’s a single study, but it’s another reason more isn’t better.

I didn’t take folic acid before I found out I was pregnant. Should I worry? Start now and tell your provider. Spina bifida affects about 1 in 2,857 U.S. births, according to the CDC, so the vast majority of pregnancies are not affected. Your provider can talk with you about prenatal screening.

Should my partner take folic acid too? Men need folate for their own health, but a large NIH trial found that high-dose folic acid and zinc supplements did not improve sperm counts or pregnancy rates. A folate-rich diet is usually enough for him.

Sources

This article is for general information and is not a substitute for advice from your health care provider.